This invention relates to a weighing machine of the so-called multi-point cell type, characterized as having a weighing table supported by a plurality of scale cells and adding the weight signals from these scale cells to measure the weight of a target object placed on the weighing table. In particular, this invention relates to such a machine capable of high-speed weighing.
Weighing machines of the multi-point cell type, as disclosed in Japanese Patent Publication Tokko 63-285941, have been in use for measuring the weight of a large or heavy target object, such as an automobile, by supporting a weighing table by a plurality of weighing cells and adding the weight signals outputted from the individual cells. Compared to weighing machines with only one cell adapted to measure the entire weight of a target object, weighing machines of the multiple-point cell type are advantageous because the size of the weighing table can be made larger since many weighing cells of about the same size can be used to support it. The scale cells are typically comprised of a deformable body with strain gauges attached thereon.
When such a weighing machine is used to measure the weight of a target object moving on a weighing conveyor, mechanical vibrations are caused to the plurality of scale cells due, for example, to the shock associated with the shift of the weight of the target object from a conveyor for its transportation to the weighing conveyor. In view of the above, it has been known, as shown in FIG. 13, to use amplifiers 22, filters (with cutoff frequency 5 Hz-10 Hz) 27 and A/D converters 24 to process weight signals outputted from a plurality of scale cells 1 and 2, to add these processed signals together by a CPU 25 and to thereafter extract a filtered signal with its high-frequency vibration components attenuated. This process is illustrated in FIG. 14.
It takes, however, a significantly long time (referred to as the stabilization time S), say, of about 200 msec until such a filtered signal becomes stabilized, and this has indeed been a source of problem in the attempt to achieve high-speed weighing. In view of the above, it has been known to carry out a stabilization routine on weight data in order to speed up a weighing process. For example, if there is a set of time-sequenced signal values a.sub.1, a.sub.2, a.sub.3, a.sub.4, a.sub.5, a.sub.6, . . . , a final equilibrium value to which the signal values are expected to converge, may be predicted by first taking the average of a.sub.1, a.sub.2 and a.sub.3, next taking the average of a.sub.2, a.sub.3 and a.sub.4, then taking the average of a.sub.3, a.sub.4 and a.sub.5 and so forth. In this manner, a partially stabilized value can be obtained within a shorter time period, say, of about 150 msec without waiting for the full stabilization time S. The overall weighing efficiency is thereby improved if such partially stabilized values are used as the stabilized weight value.
There are situations, however, where scale cells pick up the effects of environmental vibrations (to be referred to simply as "the floor vibrations") such as vibrations of the ground, building, floor and/or table on which they are set. In general, the floor vibrations have frequencies lower than those of the weighing system. Thus, they are passed through the digital filter 27 and added to the weight signals which are outputted, as shown in FIG. 15. If the averaging method described above were applied to this situation by considering time-sequenced signal values b.sub.1, b.sub.2, b.sub.3, b.sub.4, b.sub.5, b.sub.6, . . . , the convergence would be slow because of the effects of these low-frequency components. If the cutoff frequency of the digital filters 27 is lowered in an attempt to reduce the effects of the floor vibrations, however, this will tend to increase the stabilization time S, adversely affecting the capability of the system as a whole.
Another problem to be taken into consideration is the difference in sensitivity among the plurality of scale cells, due, for example, to the fluctuations in the resistance of the strain gauges. In other words, sensitivity of each scale cell must be adjusted. Japanese Patent Publication Tokkai 63-282616 taught the technology of preliminarily measuring the sensitivity of each scale cell, storing it in a memory device and correcting weight data from the scale cells by using the stored sensitivity characteristics. According to this technology, however, it is necessary to apply the same load to all scale cells. After a weighing table is attached to the scale cells, it is practically impossible to apply the same load equally to all scale cells when the object to be weighed is incorrectly positioned.